In alchemy, everything is a circle, Fire -> Metal -> Earth -> Water -> Poison -> Wood -> Fire (with Soul in its center, which everything makes stronger.)
If the main element is the start point, the one to its left is its fuel, and the one to its right is its output.
The one directly across from it is the energy inverse.
Fire -> <- Water, Metal -> <- Wood, Earth -> <- Poison, Soul -> <- Soul.
Element insults its "inverse" (1-1=0), weakens its "fuel" (2-1=1), and strengthens its "output" (1+1=2).
I hope that helped.